Heavy rain events occur in both rural and urban areas. In rural settings, the ground is often able to absorb a substantial percentage of the precipitation to the point where the ground becomes saturated. Then runoff occurs, with consequent loading of municipal infrastructures, if present. In urban environments, the surface on which the precipitation falls is often man-made and impermeable. These relatively impermeable surfaces include parking lots, streets and roof tops which must manage nearly the entire volume of precipitation that falls onto these surfaces. While streets and parking lots have drains and systems to channel and funnel the rainfall, these systems can quickly become overwhelmed when rain events exceed one inch over a short period of time. As a result, the burden falls on the municipal wastewater or sewer system to manage the great majority of the effluent.
Rooftops in particular retain and transfer almost 100% of the rain during a rain event. New building codes often require that commercial roofs need to retain a specific volume of water per unit area of the roof. A system located either directly on the roof or in a separate structure, is therefore required to store this volume and slowly release this volume into either the environment, a wastewater facility or the sewer system over a period of time in an effort to lessen the burden on these systems. The cost of added real estate for a retention pond or a cistern to temporarily store the rainfall substantially increases the cost of construction.
Against this background, it would be desirable to provide a method and system for managing rainfall that has the following features:                Has a minimal installed cost;        Is compatible with the impermeable roofing membrane;        Is capable of retaining for example 1.6″ of rainfall to meet relevant building codes;        Is capable of slowly leaching out 1.6″ of rainfall over a 24 hour period;        Requires no additional effluent retention system;        Provides some cooling or insulation to the building;        Enhances (or does not diminish) the overall appearance of the property;        Meets all building codes and requirements;        Provides additional protection against hail and the elements; and        Requires little to no maintenance.        
Conventionally, many flat roofing systems are typically surfaced with an impermeable layer. Typically the material is rubberized for ductility and is supplied in rolls that are seamed together with specialty primers and adhesives. These materials have exhibited durability to cold, heat and UV light. The manufacturers generally warranty such roofing systems for 20 years. When installed properly, the roof is impermeable and graded to channel water into drains or spouts that transfer the water from the roof surface.
In recent years, it has been shown that installing a secondary system over the primary rubberized roofing material can provide added benefits to building construction and maintenance costs.
Ford's Rouge Factory in Dearborn, Mich. installed a sedum terrarium on the roof that both absorbs excess rainfall and provides a heating and cooling benefit to the facility. Natural grass and other varieties of plants can also serve a similar function. More light is absorbed by the plants during summer months and evaporative cooling lessens the burden on facility cooling systems. During winter months, the material acts as an added layer of insulation. However, these organic materials can become a fire hazard when and if a drought causes the vegetation to die or becomes dormant during the dry spell.
A thermoplastic drainage/water retention member is often installed between the roof surface and the green roof growing medium/plants. U.S. Pat. No. 8,272,163 describes a modular interlocking pre-vegetated roof system. The system is comprised of individual trays with a plurality of reservoirs designed to retain water for plant growth. The system retains water but water is prevented from flowing through the reservoirs and onto the roof. An overflow drain is provided on the top surface when the growing medium is saturated. While the system retains a great deal of water, the reservoir retains very little water, relying mainly on the growing medium to retain the water.
An alternative system involves installing an artificial turf surface over the traditional rubberized roofing material. These systems not only enhance the aesthetic appearance of the roof but can also be engineered with infill or other materials that absorb and retain a percentage of the rainfall. They also act as the primary UV light-absorbing surface, as opposed to the rubberized roofing, and extend the life of the rubberized layer. However, these systems are generally heavy and can add several pounds per square foot to the roofing surface. They also require large amounts of infill material in order to gather, retain, and eventually evaporate enough moisture to combat a deluge of more than an inch of rain that generally taxes the municipal sewer system. Once these systems become saturated, they fail to function effectively and add substantial bearing weight to the roof.
Generally, the typical roofing system transfers nearly all the moisture that falls on the surface to the downspouts and roof drains that then transfer the burden to either a secondary water retention system or the municipal sewer system.
Among the art considered before filing this application are the following references: U.S. patent publication Nos. 2009/0188172; 2011/0030291; U.S. Pat. No. 8,272,163; European patent No. EP2322726. Also considered were the following publications: Guidelines for the Design and Construction of Stormwater Management Systems (pp. 69 & 71; FIG. 4-3); and Nophadrain Smart Green Roof Systems (Section 3.1-3.2 and “Construction of an Extensive Green Roof”).